It is not unfrequent in the process of drilling a deep well for oil or gas production to encounter formations incompatible from the drilling viewpoint, e.g. formations with abnormally high or low formation pressures, or else with crumbling or caving-in rock.
To provide for further drilling without reducing the predetermined diameter of the well, a portion of the borehole in the zone of the exposed troublesome formation is reamed, and a patcher is set into this reamed portion of the borehole, e.g. in the form of a string of profile pipes which are urged against the wall of the reamed portion of the borehole by building up excessive fluid pressure inside these pipes, followed by calibrating their internal flow passage to the required well diameter.